Combined effects of anthropogenic fires and land-use change on soil properties and processes in Patagonia, Chile

Fire and land-use change are two major types of disturbances that strongly affect the structure and function of forest ecosystems around the world, although their impacts can be difficult to quantify due to concomitant changes in climate or other land-use change factors. In this study we examined how fire and subsequent land-use conversion impacted soil properties (i.e. organic matter (OM), total available pools of carbon (C), nitrogen (N), and phosphorous (P)), and processes (i.e. N cycling inferred through δ15N) in each of six different land cover types, including old- and second-growth native Nothofagus pumilio, ∼50 year old exotic conifer plantations, and grassland pastures. We selected six land cover types, including unburned old-growth (MF), and post-fire second-growth (SG) forests of N. pumilio, post-fire afforestations of Pinus contorta (PC), P. ponderosa (PP) and P. sylvestris (PS), and post-fire grassland (GR), in three watersheds in the Aysén Region, Chilean Patagonia. In one growing season, at each of 5-7 sampling locations within each site, two 10 cm deep soil cores were removed using a 12.4 cm diameter PVC soil-corer. From each soil core, organic matter content, total C and N concentrations, availability of NH4+, NO3−, and PO4−, and δ15N were determined. Additionally, we collected foliage of Osmorhiza chilensis, a forb that was present in every tree-cover condition, for δ15N determination. Unburned old-growth Nothofagus forests showed significantly higher stocks of OM, C, N and P than the P. contorta and P. sylvestris afforestations but not higher than Nothofagus second-growth forests and P. ponderosa afforestations. Conifer afforestations showed significantly lower NH4+ values than unburned Nothofagus forests, whereas no differences in NO3− were found among the land cover types. Contrary to expectations, conifer afforestations showed significantly higher plant and soil δ15N values than the unburned Nothofagus forests. Although most land cover types resulted in significant alteration of soil properties and processes relative to the mature, unburned N. pumilio forests, we highlight that P. ponderosa afforestations generated the most similar characteristics, suggesting a utility of this species to restore some ecosystem properties.